A fluid filled tube or vessel formed with a constriction or narrowing can be analysed to measure the magnitude of the constriction or narrowing.
An example of a fluid filled tube or vessel formed with a constriction or narrowing is a blood vessel having a stenosis. Assessment or measurement of the constriction can result in a useful parameter to gauge the extent of the constriction.
A standard methodology for assessment of a constriction in a fluid filled tube such as a coronary stenosis is fractional flow reserve (FFR). This technique measures the drop in pressure at two points along a vessel; see FIG. 1 of the accompanying drawings, under conditions of maximal achievable hyperaemia in a coronary environment. The Pd measurement comes from a pressure sensor on the wire and the Pa measurement comes from the catheter. A comparison is then made by expressing the mean distal pressure (Pd), as a proportion of mean proximal pressure (Pa), wherein the values are mean Pa and Pd over the entire cardiac cycle, taken over at least one complete cardiac cycle (but usually an average of 3 or more beats):
      Fractional    ⁢                  ⁢    Flow    ⁢                  ⁢    Reserve    ⁢                  ⁢          (      FFR      )        =            P      d              P      a      
Conditions of maximal hyperaemia are usually only achievable by administration of potent vasodilators such as adenosine or dipyridamole. Such vasodilators are necessary to minimise resistance from the distal vascular bed to accurately estimate the drop in pressure across a stenosis. It would be preferable not to have to use vasodilators.
Distal pressure arises from resistance of the microcirculation, in addition to active compression of small microcirculatory vessels which permeate the myocardium. When flow is measured simultaneously at different sites, it is possible to separate the pressure components arising from the distal myocardium (backward-originating pressure), from those arising from the proximal end (forward-originating pressure),
            dP      +        =          1              2        ⁢                  (                      dP            +                          ρ              ⁢                                                          ⁢              c              ⁢                                                          ⁢              dU                                )                                dP      -        =          1              2        ⁢                  (                      dP            -                          ρ              ⁢                                                          ⁢              c              ⁢                                                          ⁢              dU                                )                    
where dP is the differential of pressure, p=density of blood, c=wave speed, and dU is the differential of flow velocity.
P+ isolates forward originating pressure by removing the backward-originating component, and therefore negates the need for administration of vasoactive agents such as adenosine. Thus by comparing the ratio of P+ on either side of a stenosis it is possible to estimate stenosis severity without requiring maximal hyperaemia to be achieved. The isolated forward pressure ratio is expressed as:
      Forward    ⁢                  ⁢    pressure    ⁢                  ⁢    ratio    =            P              +        distal                    P              +        proximal            
Whilst the forward pressure ratio offers a considerable step forward as administration of vasodilator compounds are not required, it requires flow velocity to be measured in addition to pressure. This requires considerable extra skill, additional hardware and added expense.
It is an object of the invention to provide an apparatus and method of assessing a narrowing in a fluid filled tube which does not require a measurement of flow velocity, fluid flow rate, in addition to pressure measurement.
One aspect of the present invention provides a method of assessing a narrowing in a fluid filled tube having a fluid flow pressure wave having a backward-originating pressure component and a forward-originating pressure component without taking a flow velocity measurement, comprising: taking pressure measurements in the tube; separating the pressure components into the backward-originating pressure component and the forward-originating pressure component; identifying a time window when the differential of flow velocity (dU) is minimal or absent; and deriving the backward and forward pressure components for pressure measurements taken in at least the time window.
Another aspect of the present invention provides an apparatus to assess a narrowing in a fluid filled tube having a fluid flow pressure wave having a backward-originating pressure component and a forward-originating pressure component without taking a flow velocity measurement, the apparatus comprising: a pressure measurement device operable to take pressure measurements in the tube; and a processor operable to separate the pressure components into the backward-originating pressure component and the forward-originating pressure component; identify a time window when the differential of flow velocity (dU) is minimal or absent; and to derive the backward and forward pressure components for pressure measurements taken in at least the time window.
A further aspect of the present invention provides a processor configured to assess a narrowing in a fluid filled tube having a fluid flow pressure wave having a backward-originating pressure component and a forward-originating pressure component without taking a flow velocity measurement, the processor: analysing pressure measurements taken in a tube; separating the pressure components into the backward-originating pressure component and the forward-originating pressure component; identifying a time window when the differential of flow velocity (dU) is minimal or absent; and deriving the backward and forward pressure components for pressure measurements taken in at least the time window.
A yet further aspect of the present invention provides a data storage medium carrying a computer program to assess a narrowing in a fluid filled tube having a fluid flow pressure wave having a backward-originating pressure component and a forward-originating pressure component without taking a flow velocity measurement, the program: analysing pressure measurements taken in a tube; separating the pressure components into the backward-originating pressure component and the forward-originating pressure component; identifying a time window when the differential of flow velocity (dU) is minimal or absent; and deriving the backward and forward pressure components for pressure measurements taken in at least the time window.